AAdenine -- One of the four bases that make up the letters ATGC in DNA. Adenine
("A") always pairs with thymine ("T").

Allele -- One of many possible forms of a given gene. Different alleles produce
variation in inherited characteristics such as hair and eye color. In an
individual, one form of an allele may be expressed (dominant) while the other
form may be masked (recessive).

Amino acids -- There are 20 different amino acids, which are often called the
"building blocks" of proteins. Amino acids link together in long chains to form
proteins in the body.

Antibody -- A blood protein produced in response to an antigen (any substance
capable of producing antibodies) in the body. Antibodies bind to antigens,
often produced by disease, and help the body fight against them. Antibodies can
help the body to develop immunity to a disease.

Antisense -- A strand of DNA that is not responsible for coding. The antisense
strand serves as the template for RNA synthesis.

Autosome -- Any chromosome other than a sex chromosome. Humans have 22 pairs of
autosomes.

BBase pair -- A pair of bases, or molecules (either adenine and thymine or
cytosine and guanine), which forms one rung in the DNA ladder. Bases are the
letters that spell out genetic code.

Birth defect -- A physically or mentally disabling condition present in an
individual at birth. There are more than 4,000 known birth defects, some of
which are fatal. A birth defect may be caused by a faulty gene or by
environmental factors during pregnancy, such as radiation exposure.

BRCA1/BRCA2 -- Two mutant genes that are responsible for approximately 50
percent of all inherited forms of breast cancer. Normally these genes are tumor
suppressors, which limit the growth of tumors. When mutated, these genes fail
to do their job correctly, allowing cancers to grow.

CCarrier -- A person who possesses one copy of a mutant allele that can cause
disease when paired with another mutant allele. Carriers do not have the
disease they carry but can give the disease to a child if the other parent is
also a carrier.

Centromere -- The middle portion of a chromosome where two sister chromatids
(chromosome strands) join together.

Chromosome -- A DNA molecule containing genes and found in the nucleus of a
cell. Humans have 23 pairs of chromosomes (46 in all). Forty-four of our
chromosomes are autosomes and the remaining two are sex cells. Other animals
have different numbers of chromosomes. Every child gets half of his or her
chromosomes from the father and half from the mother.

Cloning -- The process of copying a specific piece of DNA. Geneticists use
cloning in their research to study genetic structure and function.

Codon -- A sequence of three bases in DNA or RNA that specifies an amino acid
or a stop signal in protein synthesis.

Cystic fibrosis -- A hereditary disease in which the primary symptoms are
breathing difficulties and respiratory infections due to an accumulation of
mucus in the lungs. The symptoms of this disease usually develop in
infancy.

Cytosine -- One of the four bases that make up the letters ATGC in DNA.
Cytosine ("C") always pairs with guanine ("G").

DDeletion -- A kind of genetic mutation that results when a chromosome loses a
piece of DNA, leading to disease or other abnormalities.

Deoxyribonucleic acid (DNA) -- The chemical found in the nucleus of the cell
that holds the genetic instructions for making all living organisms.

Diploid -- The number of chromosomes in most human cells, except sex cells.
This number is 46.

DNA - The chemical inside the nucleus of a cell that carries the genetic
instructions for making living organisms.

DNA chip -- A part of the microarray technology scientists use in gene
sequencing to understand how the thousands of genes in a genome interact
simultaneously.

DNA replication -- The process by which the DNA molecule unwinds and makes an
exact copy of itself.

DNA sequencing -- The process by which scientists determine the exact order of
the base pairs in a segment of DNA.

Dominant -- A gene that almost always results in the expression of a certain
characteristic in an individual, even though an individual's genome only
possesses one copy of the gene. This gene always has a 50-50 chance of being
passed on to a child.

Double helix -- The twisted ladder structure of the DNA molecule. The sides of
the ladder are made of sugar and phosphate molecules; the rungs consist of
nucleotide bases joined by hydrogen bonds.

Duplication -- A type of mutation that occurs when one or more copies is made
from any piece of DNA, a gene, or a chromosome.

EEnzyme -- A protein that encourages biochemical reactions. Missing or abnormal
enzymes can result in diseases and abnormalities.

Exon -- The part of a gene that contains the code for producing the gene's
protein. In humans, exons are often separated by long regions of DNA called
introns, or "junk DNA," that have no apparent function.

FFragile X syndrome -- A frequent genetic cause of mental retardation. A
mutation causes a repetition of the letters CGG in a gene on the X chromosome.
The more repeats of this code on a person's X chromosome, the more impaired he
or she is likely to be.

Genotype -- Genetic information that does not show outward characteristics.

Guanine -- One of the four bases that make up the letters ATGC in DNA. Guanine
("G") always pairs with cytosine ("C").

HHaploid -- Half the diploid number of chromosomes in sperm or egg cells (23
cells).

IIntron -- A sequence of DNA that does not code. Introns are sometimes called
"junk DNA."

KKnockout -- The process by which genes are made inactive in a laboratory.
Scientists often use knockouts in mice in order to study what effect the
inactivity of a given gene will have on the animal.

MMessenger RNA (mRNA) -- The template for protein synthesis. Each set of three
bases, or codons, in mRNA orders the production of a certain protein. The
structure of mRNA is the same as one strand of DNA.

NNon-coding DNA -- One of the two identical strands that make up the DNA double
helix. The non-coding strand, or antisense strand, is a mirror-image of the
coding strand, but does not carry the information needed to order the
production of proteins.

Nonsense mutation -- A substitution of one of the bases of DNA that results in
a stop codon.

Nucleotide -- One of the building blocks of DNA and RNA. A nucleotide is
comprised of one base (either adenine, guanine, cytosine, and thymine), one
molecule of sugar, and one molecule of phosphoric acid.

Nucleus -- The structure in the center of each cell that holds chromosomes.

Pp53 -- A gene responsible for regulating the growth cycle of a cell. A mutation
to this gene can cause certain types of cancer.

Phenotype -- Traits, such as hair and eye color and disease, that are
observable externally. These traits are not always inherited.

Polydactyly -- A genetic abnormality that causes a person to be born with more
than ten fingers and/or toes.

Ribonucleic acid (RNA) -- A chemical that is very similar to one strand of the
DNA double helix. RNA delivers important genetic messages to the cytoplasm of
cells, where proteins are made. In RNA, the letter "U" replaces the letter "T"
in DNA ("U" stands for Uracil).

SSex chromosome -- One of the two chromosomes that specify an individual's sex.
Humans have two sex chromosomes (X and Y). Females have two X chromosomes and
males have one X chromosome and one Y.

Sex-linked -- Located on the X chromosome. Sex-linked diseases are only seen in
men.

TTay-Sachs -- An incurable recessive genetic disorder that usually develops
within the first year of a child's life, causing the build-up of fat deposits
in the brain. Children born with Tay-Sachs die before the age of ten.

Thymine -- One of the four bases that make up the letters ATGC in DNA. Thymine
("T") always pairs with adenine ("A").

Note: This glossary is adapted, with permission, from the National Human Genome
Resource Institute's "Glossary of Genetic Terms." To visit the full glossary,
visit the NHGRI Web site, listed in the Resources section.